1. Field of the Invention
The present invention relates to a measuring method and measuring apparatus wherein a clearance change formed between two members is measured by projecting a light beam on the two members and by utilizing optical interference induced by reflected lights from the respective members.
2. Description of the Prior Art
The measurement of the clearance, especially minute clearance, between two objects utilizing optical interference is applied to, for example, a case of measuring the change of the minute clearance between a slider and a disk in a magnetic disk apparatus. Examples of such prior arts are disclosed in the specification of U.S. Pat. No. 4,298,283 and the official gazette of Japanese Laid-open Patent Application No. 56-103303. The measuring method disclosed in the latter is such that a light beam is projected on two members, that a regulator capable of varying the optical distance of an optical path, such as an electrostriction transducer, is installed midway of the optical path of either of reflected lights from the two members, that the irradiance of interference light based on the optical-path difference of the reflected lights from the two members is detected, that the regulator is controlled with the detected signal so as to minimize or maximize the irradiance of the interference light at all times, and that the clearance change between the two members is measured from the controlled variable of the regulator.
The measuring method, however, has the following disadvantages:
(1) An optical system is complicated. PA0 (2) In order to make the optical distance variable, a mirror or the like optical component disposed in the optical path needs to be displaced at high speed. In this regard, the regulator such as the electrostriction transducer for varying the optical distance has its mechanical resonance point determined by the geometry of the transducer. In order to raise the resonance point, the geometry of the transducer may be made small. However, the small-sized transducer exhibits a small displacement with respect to an applied voltage. For this reason, the geometry of the transducer cannot be made very small, and the resonance point cannot be made high. In consequence, when the varying speed of the clearance is high, the measurement becomes difficult.